A few simple rules you should follow to make your, and everyone else's, experience better.
Most people, such as yourself, have enough common sense to figure out how to act in a gym. (Hint: It’s pretty much the way you’d act anywhere else only sweatier). If, however, you are unsure, here is my set of Ten Gym Commandments.
1. Thou shalt not talk on the cell phone while working out. (If you are able to breath well enough to carry on a conversation you are not working hard enough.) (Sub-commandment: Thou shall not use your cell phone AT ALL in the locker room - no texting, checking Facebook, making calls, etc... Most gyms ban their use for a reason: it is way too easy for people to take pictures or videos. Respect people's privacy and enjoy a few minutes without technology.)
Whatever it is you're doing can wait a few minutes
2. Thou shalt return your weights (and towels and trash etc...) to their proper home when you are done with them. (If you can lift them in the first place, you can lift them back to where they belong.)
3. Thou shalt not sit on the equipment unless you are actively in the process of using it. (Resting is a part of working out but if you were just going to sit around on your iPhone you could have done so much more comfortably in your own living room.)
4. Thou shalt not drop your weights, or any other equipment really (except by accident, which I’d also discourage if at all possible. Toes are probably the most vulnerable body part in any gym setting.)
5. Thou shalt not sing along to whatever is on your iPod. (Understated lip syncing is fine but if you look and/or sound like you are auditioning for American Idol you should consider toning it down.)
6. Thou shalt wipe off your equipment when you are done with it. (If you’re not sure of your gym’s procedure for this – wipes, spray bottles – ask someone.)
7. Thou shalt not take up more space than necessary in the locker room. (You can spread out as much as you like in your own bedroom. Sub-commandment: Please don’t give me an annoyed look when I have to climb over you and your acre of stuff to get to my locker.)
8. Thou shalt not sit on the benches in the locker room in the nude. (Ew. Just ew.)
Towels are your friends
9. Thou shalt let the gym’s employees know if there is a problem (such as someone doing, or not doing, one of the above), something is broken, or you need assistance. Don’t be afraid to approach the gym's employees. We’re really nice people. And if someone isn’t nice to you, tell that person’s supervisor.
10. Thou shalt remember that you are not the only person in the gym and respect other people’s experience (see all of the above for how to do so.)
There are lots of others I could add but these are the main rules that will keep you and others safe and comfortable. If I had one of those “Everything I needed to know I learned in kindergarten” posters those rules would probably also apply. Feel free to add your favorite in the comments.